There have been many attempts made to produce a single crystal semiconductor compound to be used as a substrate for luminescent diodes, transistors, diodes and the like. These compounds are formed by the combination of an element of Group II or III of the Periodic Table and an element of Group VI or Group V, respectively. Since these elements have a vastly different vapor pressure at a given temperature, there is a great deal of difficulty encountered in the formation of a single crystal of the compound, particularly one large enough to be used as a substrate for the commercial, mass manufacture of semiconductor devices. Furthermore, the reproducibility of prior art processes is rather poor.
Various techniques have been employed for growing such crystals including Czochralski growth techniques and Bridgeman-Stockbarger techniques otherwise known as the vertical-gradient freeze method.
U.S. Pat. No. 3,615,203 reveals a method of forming and growing a single crystal of a Group III-V compound. However, a single crystal seed material of the Group III-V compound is not employed and the orientation of the single crystal obtained using this method cannot be predicted. A technique which can form the desired compound from its elements and grow in a single crystal of predictable orientation from a seed material is therefore desired.
Czochralski techniques for growing single crystals of III-V or II-VI compounds have met with limited success due to the decomposition of these compounds and the difference of the vapor pressures of their constituent elements.
In U.S. Pat. No. 4,083,748 I have described a method and apparatus for simultaneously synthesizing the desired semiconductor compound and growing a single crystal of the semiconductor compound comprising a Group II-VI or Group III-V compound. The method disclosed therein comprises placing a single crystal seed of the semiconductor compound adjacent a first reactant comprising a Group II or Group III element. A protective blanket, resulting from a second reactant of a Group VI or Group V element, respectively, is formed over the seed within the temperature zone to protect the seed from dissolution from the first reactant. Reactants are combined to form a melt at a first temperature within the temperature zone and to synthesize and then grow a single crystal from the melt on the seed at a second temperature within the temperature zone. By this method one can synthesize and grow single crystals of predictable orientation. However, limited results indicated incomplete synthesis resulting in waste of starting materials and in the case of gallium phosphide, for example, incorporation of metallic gallium interspersed in the grown crystal.
I have now altered the technique taught in my previous patent by changes in the apparatus and in the thermal profile used in order to obtain single crystals of III-V and II-VI compounds having relatively low dislocation densities and in many instances large areas essentially free of dislocations. Also, this new technique, which is used for growing single crystals and not synthesis of the compound, has shown the capability of being more reproducible than prior art techniques and at least after the first 3 hours, can go unattended to completion.